Singer Guy Sebastian told his former manager Titus Day to stop throwing "childish jabs" at him while Mr Day accused the celebrity of defaming him and said he looked forward to their day in court during a heated email exchange following the pair's split.

The inaugural Australian Idol winner claims Mr Day's 6 Degrees management company owes him more than $200,000 in performance fees and proceeds from a sunscreen deal he helped foster among a range of grievances he has aired in the Federal Court.

Guy Sebastian and his former manager Titus Day (left).Credit:Anthony Johnson

Mr Day is counter-suing the pop star, alleging he is owed commissions for a range of services, including brand endorsements and performances.

A bundle of exhibits accompanying an affidavit of Sebastian's filed last month shows an initially civil email exchange between the pair in February 2018, in which the singer contacted Mr Day over payment discrepancies, descend into acrimony.

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Mr Day said "given how things have been handled" 6 Degrees was keeping a fee from Gold Coast theme park Dreamworld, of which Sebastian was an ambassador, as payment for allegedly outstanding commissions.

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Sebastian said he wasn't sure what Mr Day meant by "how things have been handled": "All I'm trying to do is get some clarity on what is owed so let's stop throwing childish jabs and just get this sorted."

In response Mr Day said, "I mean how you went about defaming me and 6 Degrees to a large number of people before you unlawfully and without notice terminated the management agreement."

Sebastian denied defaming Mr Day, who he said had accepted the termination of their professional relationship.

"This was a huge and difficult decision for me and I notified the core people that I needed to because they are still people who I work with," he said, adding he had spoken highly of Mr Day personally and that he and his wife, Jules, had also sent flowers to the agency, thanking its staff.

He then accused Mr Day of "plain lying" about him violating an agreement struck at a meeting at a Paddington cafe, which was also attended by another man, Tim Freeburn, in December 2017.

"Tim will happily attest that NOTHING (sic) was agreed on in that meeting. Obviously that's why we are at where we are now," Sebastian said.

"It's not backyard cricket Titus - we don't make up rules as we go."

Mr Day replied: "You are the one who is lying or wrong on every point there and you really need to check your facts. But I'm not going to waste my time in a you-say-I-say email exchange.

"I promise you I am looking forward to my day in court with you and everyone that surrounds that."

Sebastian claims as part of the proceedings that he is owed 10 per cent of the shares and profits of Nexdius, a company that owns sunscreen brand Solar D.

He alleges in an affidavit that he was an ambassador for the brand and that he helped attach other celebrities to it, claims that Mr Day and 6 Degrees deny.